Reference may be made to the following U.S. Pat. Nos.: 2,902,542; 3,432,619; 3,435,147; 3,544,723; 3,548,107 and 3,632,888.
Pulse-Coded Modulation (PCM) is a system of audio frequency transmission whereby the continuously varying (analog) audio signal is coded into a series of digital binary pulses, transmitted over a distance, and then converted back to analog. The signal, while in digital form, may be transmitted over very large distances. Any distortion or attenuation introduced by the transmission medium can be corrected for at strategic points ("repeaters") since the full amount of information can be recovered from the digital pulse-coded signal at each repeater. Furthermore, many audio signals can be combined onto one PCM line, and later separated, so that the line can transmit many conversations. This multiplexing is achieved by sequentially transmitting one pulse from each conversation onto the line, and, at the receiving end of the line, using a demultiplexer to divert each pulse to the correct converter, each of which then converts the pulses of one conversation back to the conventional analog form. The principle is described in various publications, and reference may be made for instance to "An Introduction to PCM Switching", Automatic Electric Technical Journal, April, 1971.
Control signal information, such as the number being called, etc., is conveyed over conventional telephone lines by audio-frequency tones, such as the Touch Calling Multifrequency (TCMF) and the Two-Out-of-Six Multifrequency (2/6 MF) schemes. In these schemes two audio frequencies are generated by oscillators and applied to the conventional telephone line, and filters are used at the receiving end to detect the presence of these control signal tones and cause the system to operate accordingly.
Although many different schemes could be used to transmit the control signal information over PCM lines, it is desired, for compatibility reasons, to inject the same tones into the analog circuitry and code them, like any other audio signal, such as voice signals, into the PCM format. Then, at the receiving end, the demultiplexer could convert this pulse-coded tone back to analog, and the presence of the tones may be detected, as with conventional telephone lines, by filters, as set forth for example in U.S. Pat. No. 3,544,723. This patent also suggests the detection of a PCM tone signal by comparing an incoming signal to its delayed derivative, the detection being effected in the PCM domain.
Improved method and apparatus for detecting one or more tones in a PCM signal are disclosed in applicant's copending applications, Ser. No. 309,020, filed Nov. 24, 1972, now U.S. Pat. No. 3,863,030; and Ser. No. 311,064, filed Dec. 1, 1972, now U.S. Pat. No. 3,824,471, and assigned to the same assignee here. Those applications use digital Fourier spectrum analysis and describe two different techniques for implementation.